1. Field of the Invention
The present invention relates to tablets having diminished fire producing properties and is a slow dissolving product containing selected mixtures of hydrated calcium hypochlorite with magnesium sulfate heptahydrate.
2. Brief Description of Art
Hydrated calcium hypochlorite is a strong oxidizer and as such can cause a severe increase in the burning rate of combustible material with which it comes in contact. This oxidation characteristic can cause problems both in the transport and storage of the product. For example, fires involving calcium hypochlorite can be quite vigorous, particularly when combustible material is present, including the product's packaging material itself (e.g., plastic, cardboard). The blends of hydrated calcium hypochlorite and magnesium sulfate heptahydrate of the invention are not classified as a “Division 5.1 Oxidizer” (i.e. they do not increase the burning rate of combustible material) as measured by an internationally recognized test standard, i.e., the United Nations Protocol: Transport of Dangerous Goods: Manual of Tests and Criteria, Section 34; Classification Procedures, Test Methods, and Criteria relating to Oxidizing Substances of Division 5.1.
Products that are “Division 5.1 Oxidizers” are by definition “dangerous goods” for purposes of transport. The following references have discussed this fire-causing problem and offered solutions to it.
U.S. Pat. No. 3,793,216 (Feb. 19, 1974) (assigned to Pennwalt) teaches adding water in the form of a hydrated inorganic salt to anhydrous calcium hypochlorite (less than 1% water) so that the total resulting water in the blend was from 3 to 13% resulting in a product that “provides resistance to exothermic, self-propagating decomposition when contacted by flame, spark, or a contaminating organic substance”. The test used in this patent to verify this property was an “ignition test”, which contacts the test sample (from 10 to 500 grams) with a lit match, or a drop of glycerin or 2-propanol. Ignition and self-propagation of burning are indicators of failure. Blends that have a delayed reaction, a less vigorous reaction, or a localized reaction compared to anhydrous calcium hypochlorite were considered to be successful blends. However, this “ignition test” is not a demanding test. Since no fuel is present the procedure does not test the oxidizing properties, i.e., they could still increase the burning rate of combustible materials. In fact, as will be shown below, many blend products described by this Pennwalt patent pass this ignition test but would still be classified as “Division 5.1 Oxidizers”.
U.S. Pat. No. 4,201,756 (May 6, 1980), (assigned to Olin) teaches the coating of calcium hypochlorite with a plurality of layers of inorganic salts. The first layer must be comprised of salts of Periodic Table Group I alkali metal salts (sodium, potassium, lithium, rubidium, cesium or francium.) The salt must be a chloride, chlorate, nitrate, bromide, bromate, or sulfate. The first layer forms a barrier so that incompatible salts such as basic aluminum sulfate, alkalized magnesium sulfate, and sodium borates may be coated with a second layer on the pre-coated calcium hypochlorite without physical contact with the calcium hypochlorite. See column 17, lines 36 to 54. The layers of salt form a physical barrier, which resists dusting and degradation during handling, and also decreases propensity for ignition and self-sustained decomposition when contacted by a lighted match or incompatible organic materials.
There are many differences between this Olin patent and the present invention. First, the requirement that an inert alkaline salt (such as sodium chloride) be added as a barrier between calcium hypochlorite and a material such as magnesium sulfate heptahydrate is not necessary. Second, the requirement that the final water content of the coated calcium hypochlorite be between 0.5 and 10% is actually counter-productive for reducing the oxidizing behavior of calcium hypochlorite. As such, the mechanism is different from that disclosed in this invention, which involves physically mixing the two substances so that they are in physical contact with each other, and maintaining a water content of at least 17%.
The best indicator of the difference between the two ideas is shown using Example XIV in the Olin patent. The example shows that calcium hypochlorite encapsulated with sodium chloride (approximately 21% by weight) prevented ignition of the material when contacted with a lighted match, i.e., it failed to undergo self-sustained decomposition. Our experiments show that physically blending 21% sodium chloride with calcium hypochlorite actually accelerates burning. As stated above, the mechanism is therefore different.
In addition, calcium hypochlorite tablets have demonstrated very fast dissolving rates in water. For example, as shown in U.S. Pat. Nos. 4,876,003 and 4,928,813 (both issued to Casburg and were assigned to Olin), plastic sleeves were positioned around the tablet to slow down the dissolving rate of calcium hypochlorite tablets. When placed around the tablets, the tablets last longer, thus providing convenience in chlorinating swimming pools and other applications. However, such plastic sleeves after use, must be removed from the skimmers, feeders and floaters for the swimming pools where they were used. This removal and discarding can be inconvenient to the pool owner. Alternatively, finely divided polyfluorinated polymer has been added to calcium hypochlorite tablets to cause the tablets to dissolve slower. See U.S. Pat. Nos. 4,970,020 and 5,205,961. Other chemical blends for making calcium hypochlorite tables are disclosed in U.S. Pat. Nos. 4,145,306; 4,192,763; 4,692,335; 4,865,760; 4,961,872; 5,009,806; 5,164,109; and 5,753,602.
There is still a need in this art to produce blended calcium hypochlorite tabletted products that are (1) relatively slow dissolving product that does not require the use of a plastic sleeve or insoluble material and (2) is not classified as a Division 5.1 Oxidizer and which has enhanced safety. (i.e., diminished fire-producing properties). The present invention provides a solution to that need.